This invention relates generally to an apparatus and method for starting a motor and, more particularly, to an apparatus and method for initially soft-starting a three-phase motor using reduced-phase power, and then switching to three-phase power upon the occurrence of a triggering event.
Many commercial heating, ventilation, and air-conditioning (HVAC) systems circulate air by driving a load such as a fan, pump, or compressor with an electric motor. The motor is connected to a source of electric power. The load may be connected directly to the motor or may be connected by way of load-coupling components, such as a drive belt. Usually, such air circulation systems must be turned on and off on a regular basis to maintain the environment at a desired temperature. Because such systems are typically employed in office or residential settings, high efficiency and low noise are important.
Most of the components of circulation systems, particularly the load, load-coupling components, and motor deteriorate after continued use over a period of time. As these components deteriorate, they must be replaced at a relatively substantial cost. Therefore, other important considerations are the life expectancy and replacement cost of these fan drive components.
In some circulation systems, it is more desirable to employ a polyphase motor instead of a single-phase motor. (As used herein, "polyphase" means three phase or a higher number of phases.) Three-phase motors have many advantages over traditional PSC (permanent split capacitor) motors. For example, the frequency of the voltage delivered to a three-phase motor can be varied while maintaining a constant volts/frequency ratio. This capability allows more efficient motor operation while delivering the motor's rated torque. Other advantages of three-phase motors are: lower locked rotor currents; higher starting torque; lower full load currents; and improved reliability due to the elimination of the starting capacitor required in a PSC motor circuit. A disadvantage of a three-phase motor is its high starting torque. A high starting torque can damage the coupling components or load. For example, the high starting torque of a three-phase motor can cause belt squeal, belt slippage, coupling failure, load deterioration, vibration, motor wear, or other undesirable wear conditions. These problems particularly occur when the motor is heavily loaded at start-up. Furthermore, the efficiency of the motor is compromised by the coupling slippage that may occur as a result of the rapid acceleration of the motor. Thus, notwithstanding the many advantages of three-phase motors, in some situations, motor efficiency can decrease, noise output due to belt squeal, slippage, vibration, or coupling strain can increase, life expectancy due to wear and tear on coupling components and loads can decrease, and replacement costs of coupling components and loads can increase.
Therefore a need remains in the industry for an apparatus and method for soft-starting a three-phase motor which overcomes the potential disadvantages of three-phase motors while exploiting the advantages discussed above. The present invention as described below addresses this need.